We demonstrate a laser system with a frequency instability of 0.7 Hz for integration times of 20 s, corresponding to $2.3\times {10}^{-15\mathrm{}}$ fractional instability. This is the lowest short-term instability of any current oscillator. The system uses diode-pumped miniature Nd:YAG lasers stabilized to sapphire Fabry-Pérot reference cavities at liquid helium temperature. Cryogenic optical resonators are promising for precision experiments, such as high-resolution spectroscopy, flywheel oscillators for optical frequency standards, and displacement measurements. As an example, we report a measurement of the ultralow thermal expansion of a sapphire cavity at 1.9 K.

• Received 27 November 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.4741